The present invention relates to a method for controlling positive or negative freewheeling paths of the phases of a matrix converter having nine bidirectional power switches arranged in a 3xc3x973 switch matrix, with each power switch having two back-to-back series-connected semiconductor switches.
A matrix converter is a self-commutated direct converter which allows a rigid three-phase network to be converted to a system with a variable voltage and frequency. The arrangement of the bidirectional power switches in a 3xc3x973 switch matrix allows one of the three output phases of the matrix converter to be electrically connected to one input phase. One phase of the matrix converter comprises an arrangement of three bidirectional power switches, which are each connected on one side to an input phase and on the other side to an output phase. An arrangement such as this is also referred to as a 3xc3x971 switch matrix. The matrix converter does not require an intermediate circuit. The self-commutated direct converter offers the advantage that its topology means that it allows feedback, and appropriately applied control allows sinusoidal network currents to be achieved.
The bidirectional power switches in the matrix converter each have two back-to-back series-connected semiconductor switches. Insulated gate bipolar transistors (IGBTs) are preferably used as the semiconductor switches, each of which having a back-to-back parallel-connected diode. Bidirectional power switches designed in this way are preferably used for converters for low and medium power levels. Actuation of these semiconductor switches in the bidirectional power switches in each case produces a current path in a direction which is governed by the arrangement of the semiconductor switches. If both the semiconductor switches in one bidirectional power switch are actuated, then this power switch is switched on bidirectionally, and current can flow in both directions. This results in a reliable electrical connection between an input phase and an output phase of the matrix converter. If only one semiconductor switch in a bidirectional power switch is actuated, then this switch is switched on unidirectionally, and an electrical connection is produced only for a preferred current direction between an input phase and an output phase of the matrix converter.
Optimum actuation allows sinusoidal network current consumption. In order to avoid loading the feeding network with pulse-frequency harmonics, the matrix converter also requires an input filter, comprising LC elements. Owing to the large number of semiconductor switches, the actuation process is very complex.
When the matrix converter is switched off, it is necessary to ensure that the output current tends to zero before all the switches can be switched off. It is advantageous for the current to be reduced to zero by means of a natural diode function, rather than by current regulation. This can be achieved by means of freewheeling in the matrix converter. Furthermore it is desirable for the matrix converter to be switched to a safe state at any time in the event of a fault, for example in the event of an overcurrent. This means that it is desirable to able to change from commutation control for normal operation to freewheeling control.
The object of the commutation logic for one phase of the matrix converter is to actuate the six semiconductor switches of the three bidirectional power switches in the respective output phases of the matrix converter in such a way that the switching commands for the triggering equipment are implemented correctly, allowing reliable operation of the converter. The control logic must in all circumstances prevent a short circuit of the supply voltages on the input side, or at the output, not resulting in an interruption in the load current. Otherwise, this would lead to destruction of at least one semiconductor switch as a result of an overcurrent or overvoltage.
In the case of voltage-controlled commutation logic, the phase that is intended to be commutated to and the polarity of the phase-to-phase input voltages are required. In this case, the polarity of the output current is not important, since there is always a path for both current paths both in the steady case and during commutation.
FIG. 2 shows, in the form of a graph, all the possible commutation steps. Overall, there are 22 different switching operations which can occur, depending on the commutation control. A xe2x80x9c1xe2x80x9d means that a semiconductor switch in a bidirectional power switch is switched on, with a xe2x80x9c0xe2x80x9d representing a switched-off semiconductor switch in a bidirectional power switch.
Once the matrix converter has been switched on, all the semiconductor switches in the nine bidirectional power switches are switched off. If it is intended to change to a steady state, a change must be made from the center point xe2x80x9cOFFxe2x80x9d to one of the three corner points xe2x80x9cUxe2x80x9d, xe2x80x9cVxe2x80x9d or xe2x80x9cWxe2x80x9d. A change can be made from one steady state to any other steady state. Depending on the polarities of the voltages, there are three different routes to change to a new steady state. While traveling from one steady state to the next, it is therefore impossible to reverse. This xe2x80x9cone-way only regulationxe2x80x9d is necessary in order to avoid undefined states and reactions.
It must be possible to switch off the matrix converter at any time, even during a commutation process. To allow this to be done, there must be a route from each state to the center point xe2x80x9cOFFxe2x80x9d in FIG. 2. In order to avoid overvoltages and destruction of the semiconductor switches in the bidirectional power switches when switching off all the semiconductor switches, a device must be provided which allows current to continue to flow during the switching off process, dissipating the energy in the load.
This further current flow is made possible by means of a freewheeling path, which must be switched. If only one semiconductor switch is actuated in a bidirectional power switch with two back-to-back series-connected semiconductor switches, this means that the bidirectional power switch is closed unidirectionally. If its enabled current direction is in the opposite direction to the voltage which is applied to it, then this is referred to as freewheeling. If this unidirectionally closed bidirectional power switch allows a positive current flow, that is to say from the feed network to the load, then this freewheeling is referred to as positive freewheeling. If a negative current flow is allowed, then the freewheeling is referred to as negative freewheeling.
Four-stage, current-dependent commutation is known from the publication xe2x80x9cA Matrix Converter without reactive clamp elements for an induction motor drive systemxe2x80x9d, A. Schuster, PESC 98, Japan, pages 714-720. This current-dependent commutation uses the polarity of the output current as a decision variable for the switching sequence of the four semiconductor switches which are involved in the commutation process in the two bidirectional power switches. Furthermore, this publication describes a switching-off strategy for the matrix converter, which can change from specific states to the freewheeling mode, following normal operation.
This approach has the disadvantage that a certain time delay may occur before the freewheeling mode is reliably reached. It is therefore not possible to switch off the matrix converter at any time. Nevertheless, additional protective measures are thus required for each semiconductor switch in the bidirectional power switches for this time period. This publication proposes the use of varistors as the protective measure, which are connected electrically in parallel with each semiconductor switch.
A method for commutation and for switching on a freewheeling path is known from the publication xe2x80x9cA Matrix converter switching controller for low losses operation without snubber circuitsxe2x80x9d, R. Cittadini, J. J. Huselstein, C. Glaize, EPE 97, pages 4.199 to 4.203. Depending on the voltage which is applied to the commutation group, additional switches are switched on in addition to a bidirectional switch, and allow freewheeling. These additional switches thus allow energy which is stored in the load inductance to be fed back into the network. This commutation process is a two-stage process. In this commutation process, the semiconductor switch which is oriented from the current flow direction in the voltage direction is switched off during the transitional state, while the freewheeling diode valves (with current flow oriented in the opposite direction to the voltage direction) remain switched on continuously. In this method, the voltage is subdivided into three regions. At low voltage levels, the presetting is not influenced by the commutation control, that is to say all the semiconductor switches in the commutation group are switched on, so that this briefly results in a short circuit. Since this occurs in the transitional region where the voltages are low, the short-circuit currents are very low.
This method has the disadvantage that no freewheeling paths are kept available around the zero point, so that unreliable operation may occur. An unreliable measurement in the voltage evaluation process can lead to a phase short circuit when the voltages then become higher.
A two-step commutation strategy for a matrix converter and which is voltage-oriented is known from the publication xe2x80x9cSemi Natural two steps commutation strategy for matrix convertersxe2x80x9d, M. Ziegler, W. Hofmann, PESC 98, pages 727 to 731. This control method is based on the detection of 60xc2x0 intervals as they occur. An interval starts with an intersection of two input winding section voltages and ends with a subsequent intersection of two input winding section voltages. In consequence, none of the phase-to-phase voltages change their polarity within one interval. Three main states, which do not cause a short circuit between two input phases, may be found in each case as a function of specific intervals. Each main state produces a bidirectional connection between an output phase and the nominal input phase. In addition, so-called redundant unidirectional switches are closed in the main states. In the situation where one input phase is at a higher voltage than the nominal input phase, a corresponding unidirectional switch is closed in the reverse direction. In the situation where the voltage is lower, a corresponding unidirectional switch is closed in the forward direction. Four unidirectional switches are always closed, and two are open, in the main states in a converter element of a 3xc3x973 matrix converter. A converter element in a 3xc3x973 matrix converter comprises three bidirectional switches, which may connect each of the three input phases of the matrix converter to one output phase. The commutation of the output current from one input phase to the other can then always take place in only two steps. In the main states, each output phase is bidirectionally connected to one input phase. During the transition from one main state to the subsequent state, the bidirectional connection in a diode link to the input phase is disconnected first, with the next bidirectional state (main state) then being implemented.
In this method, it is unclear which switches can be switched off in the event of a fault, and which must be switched on. Pure freewheeling control operation is thus impossible. Furthermore, long-lasting short circuits can occur on the input voltages, if the polarity of the phase-to-phase voltages is measured incorrectly.
The method proposed in the aforementioned publication has been disclosed in DE 197 46 797.
It would therefore be desirable and advantageous to provide an improved method for controlling continuously available positive or negative freewheeling paths in a matrix converter that can be switched off without any time delay, in the event that the matrix converter has a fault or is switched off, without destroying the matrix converter. The method should also be robust with respect to measurement errors of the voltage polarity in a region around the zero point.
According to one aspect of the invention, a method is disclosed for controlling positive or negative freewheeling paths in a phase of a matrix converter having nine bidirectional power switches arranged in a 3xc3x973 switch matrix, with each power switch associated with a matrix converter phase and having two back-to-back series-connected semiconductor switches. The method includes identifying a bidirectional power switch of a matrix converter phase having a most negative line voltage; identifying a bidirectional power switch of a matrix converter phase having a most positive line voltage; actuating the semiconductor switch of the identified bidirectional power switch having the most negative line voltage to provide a positive freewheeling path capable of carrying a positive load current, and actuating the semiconductor switch of the identified bidirectional power switch having the most positive line voltage to provide a negative freewheeling path capable of carrying a negative load current.
This results in pure freewheeling control which allows commutation control for a matrix converter to be switched off at any time without destroying the matrix converter. The separate freewheeling control means that a current path which ensures that the current flowing in a load inductance flows continuously, independently of commutation control, at all times in the matrix converter, thereby always ensuring energy feedback from the load side to the network side. The freewheeling control according to the invention switches on bidirectional power switches in the matrix converter in order to protect the matrix converter. The matrix converter can then be switched off without any time delay in the event of fault or in the event of a matrix converter being switched off, without destroying said matrix converter. There is hence no longer any need for any overvoltage protection devices, such as snubber circuits for the semiconductor switches in the bidirectional power switches, for the semiconductor switches in the bidirectional power switches of the matrix converter.
In one advantageous embodiment, in a region of a positive or negative zero crossing of a phase-to-phase input voltage, the semiconductor switches in the two bidirectional power switches which are involved at the zero crossing in each phase of the matrix converter are first selected for a negative or positive freewheeling path, and these semiconductor switches are then compared with semiconductor switches in the bidirectional power switches determined by means of a commutation method. If this comparison results in a match, the selected semiconductor switch, which is also determined as a function of the commutation, is actuated. If no match is found between semiconductor switches which are switched on as a function of freewheeling and those which are switched on as a function of commutation, then the semiconductor switches in the bidirectional power switches which are to be switched as a function of freewheeling in each matrix converter phase are actuated.
The risk of incorrect measurement results is greatest in the region around the voltage zero crossing of the phase-to-phase input voltage of the matrix converter. These incorrect voltage measurements can lead to incorrect switching operations as a function of freewheeling, so that short circuits as a function of freewheeling are possible. This advantageous method means that there is no longer any risk of short circuits even when the voltage polarities are determined in this way.
In another advantageous embodiment, when a region of a positive or negative zero crossing of a phase-to-phase input voltage of the matrix converter is reached, the previously mentioned advantageous embodiment is activated immediately, with the semiconductor switches which are actuated as a function of freewheeling and were active before this region remaining active for a predetermined time. An overlap region is thus defined, in which both methods are active. One freewheeling process is thus always switched. Once the overlap time has elapsed, the chronologically older method is deactivated.
With the method of the invention, freewheeling paths are switched on continuously during operation of the matrix converter with their functions being separated from the switched-on semiconductor switches in the bidirectional power switches in the commutation control process, thus always ensuring that power is supplied on only one path from the load side to the feeding network. In the preferred embodiment of the method according to the invention, this method is robust with respect to voltage errors, particularly at the zero crossing of the phase-to-phase input voltages.
Unnecessary switching operations can be eliminated by logically linking the switching operations which are dependent on commutation and the switching operations which are dependent on freewheeling with each other by means of an OR gate. Even switches, which have been switched on by the commutation control process, are thereby protected against being switched off by the commutation control process or by other processes. The additional switches which are dependent on freewheeling have no current-carrying function during normal operation of the matrix converter. They are switched on such that, when the matrix converter is switched off, a freewheeling path is ensured in each direction for the load current. This is necessary since suddenly switching off all the semiconductor switches in the bidirectional power switches in the matrix converter can lead to destruction of the semiconductor switches in the bidirectional power switches as a result of overvoltage. The freewheeling paths which are switched by means of the method according to the invention allow current to flow continuously between one output phase and a feeding network, with the current being driven against a higher network voltage.